1. Field of the Invention
The invention relates to a photographic support material with a water-resistant coating of a polyolefin resin.
2. Brief Description of the Background of the Invention Including Prior Art
Water-resistant photographic support materials comprise a plastic film or a base paper with synthetic resin coatings applied to preferably both sides, which include polymers, for example polyethylene, and which are applied by way of extrusion coating or which comprise an organic lacquer mixture, which is placed on the paper by way of a submersion or spray method and which is dried and hardened or solidified by employing heat or energy-rich radiation.
One or several silver halides containing light-sensitive layers are applied on one of the resin layers. In case of light-sensitive layers, it can refer to both black and white as well as color photographic layers.
The synthetic resin layer (front-side coating) disposed under the light-sensitive layer or, respectively, layers comprises usually a light-reflecting white pigment as well as possibly color pigments, optical brighteners and/or other additives such as antistatic agents, dispersion additives for the white pigment, antioxidants, separating agents, and the like.
The synthetic resin layer, disposed on the paper side opposite to that of the one with the light-sensitive layers (back-side coating), can be pigmented or can be left unpigmented and/or can contain other additives, which result from the use of the laminate as a photographic support material in each case and which, in principle, can correspond to those of the front-side coating.
The front-side coating can contain additional function layers which improve, for example, the adhesion of the light-sensitive layers.
In addition, the back-side coating can be provided with additional function layers which can improve, for example, the capability of writing on the material, the antistatic property, the slip properties, the flatness (planarity), or several of these properties.
In order to achieve that a resin coated photographic paper support, after a one-side application of photographic layers, does not exhibit an undesirable curvature (curl), it is usually provided that the front-side polyethylene layer comprises substantially a low-density polyethylene (LDPE), whereas the back-side polyethylene layer predominantly comprises a high-density polyethylene (HDPE), compare German Patent Application Laid Open DE-OS No. 2,028,600.
The coating of a photographic base paper or of a film material with polyolefin coating by way of extrusion via a slot die is a method known in the art. It is also known that, during the extrusion of the polyethylene, certain difficulties occur, which can result, owing to the high sensitivity of the photographic process, in perceivable disturbances in photographic layers or even in a uselessness of the support material.
The use of low-density polyethylenes (LDPE), produced by polymerizing ethylene according to the high-pressure method, transfers the disadvantages of insufficient stiffness and hardness of this polyethylene to the photographic paper support, which can be noticed by an insufficient overall stiffness of the final product and, in particular, in case of glossy surfaces, by an increased tendency to sticking in the wound roll.
However, also the use of polyethylene of higher density (HPDE), produced by polymerization of ethylene according the low-pressure method, transfers to the photographic-paper support material disadvantages which are generated by the higher melt viscosity and the crystallinity of the high-density polyethylene or result from the additives added to this polyethylene necessary for purposes of stabilization, if this polyethylene is employed in a front-side coating. These disadvantages include a weak and insufficient adhesion on the paper support and an unsatisfactory planarity of the coated paper. In addition, the low "draw-down" of the high-density polyethylene can be a disadvantage, in particular where thinner coatings are desired.
In order to limit the disadvantages associated with the individual polyethylene types, optimized mixtures of low-density polyethylene and high-density polyethylene are usually employed, (e.g. U.S. Pat. No. 4,699,874).
It is a joint property of the two polyethylene types and of their mixtures that they can only be filled to a limited extent with pigments. It is taught in German Patent Application Laid Open DE-OS No. 3,411,681 (English language equivalent=GB No. 21 389 64) how much the sharpness of a photographic image depends on the extent of reflection of the impinging light off the white pigments. Therefore, it is an important goal of all light-sensitive support materials to improve this reflection of the impinging light. This is achieved by employing white pigments with the highest indices of refraction, such as titanium dioxide, and by maintaining as high as possible the content in white pigment in the polyolefin resin with a very good dispersion such that a dense pigment packing is generated and present in the support near the surface. It has to be avoided in this context to generate pigment agglomerates in the polyolefin resin, since they decrease on the one hand the total light reflection and, on the other hand, they can result in disturbances and interferences during the drawing of the melt film in the extrusion coating, or later during the casting of the resin coated support with light-sensitive emulsions.
In practice of extrusion coating, the pigment concentration in a polyethylene resin coating mixture has been limited hitherto to less than 20 weight-percent and is usually between 10 and 17 weight-percent. This holds both for a pure low-density polyethylene as well as for the described mixtures of low-density polyethylene and high-density polyethylene.
The various problems associated with all photographic paper support materials coated with polyethylene have resulted in further specific developments. Such developments are for example described in the German Patent Application P No. 3,627,859.9. According to that patent application, the sum of the associated disadvantages is clearly improved on in several aspects. However, the results are not satisfactory in that the titanium-dioxide concentration is limited to about 20 percent and that the number of the pigment agglomerates is only slightly decreased.